What Does inHg Mean in Weather and Atmospheric Pressure?

InHg is the abbreviation for “inches of mercury,” a unit of atmospheric pressure widely used in weather reports, aviation, and engineering across the United States. It describes how high a column of liquid mercury would rise inside a glass tube when atmospheric pressure pushes down on an open reservoir of mercury at the base. At sea level under standard conditions, that column stands 29.92 inches tall, and meteorologists treat that figure as the baseline around which everyday pressure readings fluctuate.

Where the Unit Comes From

The inch-of-mercury unit traces back to a single experiment in the 1640s. Evangelista Torricelli, an Italian physicist, sealed one end of a glass tube, filled it with mercury, and inverted it into an open dish of the same liquid metal. The mercury column dropped partway and then stopped, held up by the weight of the atmosphere pressing on the dish. Torricelli was the first to build a mercury barometer and recognize that the column height directly reflected the pressure of the surrounding air.

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Mercury turned out to be an ideal fluid for the job. Because it is so dense, the column is a manageable length: about 30 inches at sea level rather than the roughly 34 feet you would need if you used water. The inch scale carved alongside the tube became the natural way to express pressure readings, and the convention stuck in countries that use imperial measurements. Most of the rest of the world has moved to hectopascals (hPa) or millibars (mb), which are numerically identical to each other, but inHg remains the dominant unit in U.S. weather broadcasts, home barometers sold in North America, and all aviation altimeter settings in the United States and Canada.

How to Read a Barometric Pressure Number

When a weather report says the barometric pressure is 30.15 inHg, it means the atmosphere is exerting enough force to push a mercury column up to 30.15 inches. That is slightly above the standard sea-level value of 29.92, so conditions are a touch above average. Most day-to-day readings in the contiguous United States fall between roughly 29.70 and 30.40 inHg. During a strong hurricane, pressure can drop below 28.00 inHg, and during a powerful winter high-pressure system, it can climb above 30.70.

There is one important subtlety: the pressure reported by your local TV meteorologist is almost always adjusted to sea level, regardless of your actual elevation. A city at 5,000 feet above sea level has genuinely thinner air overhead, so its raw station pressure is lower than a coastal city’s. To make readings comparable across locations, weather stations correct their measurements to what pressure would be at sea level. That is why you can meaningfully compare a barometric reading in Denver to one in Miami. If you buy a home weather station, it will typically ask you to enter your elevation during setup so it can apply that same correction.

Rising Versus Falling Pressure and What It Means for Weather

The raw number matters less than the direction it is heading. A falling barometer, meaning the inHg reading is dropping, generally signals that a low-pressure system or front is approaching. Low-pressure areas are associated with rising air, cloud formation, and precipitation. A rapidly falling reading, say a drop of 0.06 inHg or more per hour, often precedes strong storms.

A rising barometer signals the arrival or strengthening of a high-pressure system, which pushes air downward and discourages cloud development. Clearing skies and dry conditions tend to follow. A steady reading that holds within a narrow range for hours suggests the current weather pattern is not changing quickly. Experienced sailors and outdoors enthusiasts have used these trends for centuries, well before satellite forecasts, simply by watching how fast and how far the mercury column moved.

InHg in Aviation

Pilots depend on barometric pressure more than almost anyone outside of meteorology, and inHg is the unit they use every day in the U.S. and Canada. Before takeoff and during approach, pilots receive an “altimeter setting” from air traffic control, expressed in inHg to the hundredths place (for example, 29.97). They dial this number into the altimeter, a cockpit instrument that converts barometric pressure into an altitude reading. Without the correct setting, the altimeter could be off by hundreds of feet, which is dangerous when flying near terrain or other aircraft.

The relationship between pressure and altitude is roughly linear near the surface: a decrease of about 1 inHg corresponds to roughly a 1,000-foot gain in altitude. This approximation breaks down at high altitudes where the atmosphere thins unevenly, but it is a handy rule of thumb below about 10,000 feet. Because pressure changes with both altitude and weather, pilots need updated altimeter settings frequently, especially during approach. Research on altimeter design has long emphasized that combining digital readouts with analog pointer displays gives pilots the clearest picture of both their exact altitude and how quickly it is changing.

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Converting Between Pressure Units

If you read weather data from an international source, you will run into hectopascals or millibars instead of inHg. The conversion is straightforward: 1 inHg equals about 33.86 hPa (or mb). Standard sea-level pressure, 29.92 inHg, converts to 1013.25 hPa. Going the other direction, divide the hPa value by 33.86 to get inHg.

You may also encounter millimeters of mercury (mmHg), which is common in medical contexts for measuring blood pressure. One inHg equals exactly 25.4 mmHg, because there are 25.4 millimeters in an inch. So standard sea-level pressure in mmHg is about 760 mmHg. A third unit, the atmosphere (atm), defines 1 atm as exactly 29.9213 inHg, making it nearly synonymous with the standard sea-level value. These are all different yardsticks measuring the same physical quantity. The unit you encounter depends entirely on what field you are in and what country you are in.

The Daily Pressure Cycle Most People Never Notice

Even on a perfectly calm, cloudless day with no weather systems moving through, barometric pressure is not constant. It follows a subtle twice-daily rhythm called the atmospheric semidiurnal tide. Pressure peaks around 10 a.m. and 10 p.m. local time and dips around 4 a.m. and 4 p.m. The swing is small in mid-latitudes, typically around 0.03 inHg or less from peak to trough, so you would never feel it. In the tropics, the oscillation is larger and more consistent, sometimes reaching 0.06 inHg or more.

This rhythm is driven by solar heating. As the sun warms the atmosphere, ozone in the stratosphere and water vapor in the lower atmosphere absorb radiation at different rates, setting up pressure waves that propagate around the planet. The diurnal (once-daily) component ends up being relatively weak at the surface because most of its energy gets trapped higher up in the atmosphere, while the semidiurnal (twice-daily) component is the one that shows through clearly in surface readings.

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Tropical meteorologists pay close attention to these tides. In regions where day-to-day pressure changes from weather systems are small, the regular tidal signal is one of the strongest features in a barometric trace. Diurnal and semidiurnal pressure variations in the tropical Atlantic interact with surface winds and can influence patterns of open-ocean rainfall.

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Can You Feel Barometric Pressure Changes?

Many people insist they can feel a storm coming in their joints or their head, and the science partially backs them up. A narrative review of the research on barometric pressure and headache identified several proposed mechanisms: changes in atmospheric pressure may excite neurons in the trigeminal system (the nerve pathway involved in many headache types), alter blood vessel tone, or create mild pressure imbalances in the sinuses and middle ear.

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Animal research has added a concrete detail: when atmospheric pressure was experimentally lowered, specific neurons in the trigeminal nucleus of rats increased their firing rate, particularly neurons that receive input from the eye region. The researchers suggested that similar mechanisms could contribute to headache during weather changes in humans.

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The effect is real, but modest and highly individual. Not everyone is sensitive, and the pressure drop associated with an approaching storm front, maybe 0.15 to 0.30 inHg over several hours, is small in absolute terms. If you are one of the people who consistently gets a headache before rain, you are not imagining things, but tracking the barometer is unlikely to let you prevent it because the pressure change is already underway by the time you notice the reading dropping.

How Modern Barometers Measure Pressure Without Mercury

Almost no one uses an actual column of mercury anymore. Traditional mercury barometers are fragile, toxic if broken, and impractical for portable use. The aneroid barometer, invented in the 19th century, replaced mercury with a small sealed metal capsule that flexes as external pressure changes. A mechanical linkage amplifies that tiny flexing motion and drives a needle across a dial marked in inHg (or hPa, depending on the market). Many of the round, wall-mounted weather instruments you see in homes are aneroid barometers.

Digital weather stations and the barometric sensors in your smartphone use a different approach entirely. Microelectromechanical systems, or MEMS, sensors measure pressure by detecting how a tiny silicon membrane deforms under atmospheric force. Some advanced designs use piezoresistive cantilevers paired with a small air chamber to detect extremely subtle pressure changes, pushing sensitivity well beyond what older instruments could achieve.

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These MEMS sensors are so small and cheap that they now sit inside phones, fitness watches, and car tire-pressure monitors. Your phone’s weather app can report a local barometric reading in inHg without any external hardware. The unit is the same one Torricelli would recognize; the technology producing the number has just changed dramatically.

Barometric Pressure and Sea Level

Atmospheric pressure does not just push down on mercury. It pushes down on the ocean surface too, and changes in that pressure cause the water to rise or fall in response. This is called the inverse barometer effect: when atmospheric pressure drops by a certain amount, the sea surface rises to compensate, and vice versa. The relationship works out to roughly one centimeter of sea-level change for every one hectopascal (about 0.03 inHg) of pressure change.

This effect has real consequences. Along the northeast coast of North America, the inverse barometer effect accounts for roughly a quarter of the year-to-year variability in sea level measured by tide gauges. During one recent extreme event of sea-level rise along Atlantic Canada and New England, the pressure effect explained about half of the observed magnitude.

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Storm surges are the most dramatic example. When a hurricane or intense extratropical cyclone moves over coastal waters, its extremely low central pressure allows the sea surface to bulge upward, adding to the wind-driven surge. Ocean models that include the inverse barometer effect produce better storm-surge forecasts. An ensemble forecasting experiment that incorporated this effect during a severe North Sea storm (Storm Xaver in December 2013) provided stronger and earlier warning of the maximum surge level than simulations that left it out.

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Over longer time scales, persistent shifts in average atmospheric pressure patterns can even show up in multidecadal sea-level acceleration records, contributing an estimated 10 to 30 percent of the observed acceleration in parts of the Mid-Atlantic Bight and Southern New England.

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Why the U.S. Still Uses Inches of Mercury

The persistence of inHg in American weather and aviation is mostly a matter of institutional inertia. The National Weather Service issues its public forecasts using inHg because that is what American listeners expect and what American instruments display. Aviation is even more conservative about unit changes because any inconsistency between what a pilot reads, what a controller says, and what an altimeter displays creates a safety risk. International aviation outside North America uses hectopascals for altimeter settings, but switching the entire U.S. and Canadian system would require retraining tens of thousands of pilots and controllers, recertifying instruments, and updating decades of procedures simultaneously. Nobody wants to be in the middle of that transition.

For practical purposes, the unit you use does not change the physics. Whether you call standard sea-level pressure 29.92 inHg, 1013.25 hPa, or 760 mmHg, you are describing the same force. If you travel internationally or read weather data from European or Asian sources, knowing the conversion factor of roughly 33.86 hPa per inHg is all you need to translate between systems. Most weather apps will switch units in their settings menu, making the whole question academic for the casual reader. But understanding what that number represents, a measure of the weight of the air above you, connects you to a measurement tradition that has barely changed in concept since Torricelli’s glass tube first caught the light in a Florentine laboratory nearly four centuries ago.